When is a child too young to remember a trauma, and is that even the right question to ask?

"It's more about the family than the child," said Dr. Alan E. Kazdin, professor of psychology and child psychiatry director of the Yale University Parenting Center and Child Conduct Clinic.

"The child doesn't know the meaning of the stabbing and may not see blood or know what it was," said Kazdin. "The easier part to address is that the mother will have a huge traumatic reaction to this and it will likely change the interaction with the 3-year-old.

"You can't fault the mom for anything, but depressed moms are less engaged with their children."

The victims' parents – Marina and Kevin Krim, an executive with CNBC – will surely have a psychologically challenging recovery ahead, say medical experts. And their surviving child Nessie's emotional health will be intertwined with theirs.

Children always follow their parents' lead, according to Kazdin. "When a child falls on the pavement, they cry for just a second, then they look to the parent. When they see the parent isn't crying, they stop."

"This is an event the child can't really experience – like 9/11," he said. "The family will talk and cry about it and not the first, but the enduring events the family will go through in their normal reaction will be devastating to the child.… The trauma experience is not going to be a one-shot thing."

Marina Krim demonstrated her devotion to her three children in photos and daily anecdotes that she posted over the last two years on a blog, "Life with the Little Krim Kids" on LiveJournal. It was taken down after the murders.

Police said that Ortega, who is 50 and worked in the family home for more than year, remains on a breathing tube after being rushed to the hospital. She was a naturalized American born in the Dominican Republic where the Krims had visited Ortega's family in February.
Neighbors who had known the woman for years said she had no history of mental illness, and police had no motive.

Christine A. Courtois, a counseling psychologist and the author of "Treatment for Complex Trauma," said that Nessie's mother will need "extensive support" going forward. "They have to face loss, betrayal, and in addition a trial."

For the parents, it's not just about the death but the massive betrayal of responsibility," she said.

The motivations of Ortega may never be known. "It depends on the character of this woman and what set it off," said Courtois. "Something may have happened with the children. She may have been resentful about the wealth of the family or have her own history of abuse or something that unhinged her that day."

Other parents need to be vigilant and take it in the sense that this could happen to anyone," Courtois said. "Unfortunately, no one is immune."